1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, and in particular, relates to an apparatus capable of reading front and back faces of an original.
2. Description of the Related Art
A mechanism in an image reading apparatus such as a double-side image scanner of the related art will be described with reference to FIGS. 7 and 8. Here, FIG. 7 is an explanatory view which illustrates an example of a reading mechanism portion 300 in an image reading apparatus of the related art. FIG. 8 is an explanatory view which illustrates an image signal processing circuit included in the image reading apparatus of the related art.
As illustrated in FIG. 7, the reading mechanism portion 300 includes an original base plate 301 to perform reading of originals D and a sheet feeding roller 303 to feed the originals D to the original base plate 301 from an upward angle for continuous reading and double-side reading of the originals D.
An optical reading unit 309 to read a back face of an original is arranged above the original base plate 301. A movable optical reading unit 305 to read a front face of an original is arranged below the original base plate 301. The upper and lower faces of the original D fed from the sheet feeding roller 303 are read by the abovementioned optical reading units 305, 309. In the following, the front face denotes the lower face side of the original D (i.e., the lower side face of the original D in front view of FIG. 7) and the back face denotes the upper face side (i.e., the upper side face of the document D in front view of FIG. 7). Double-side reading is performed on the original D.
A platen 313 doubling as a white reference used as the white reference for the back face of the original D is arranged below the original base plate 301 in front view so as to be opposed to the optical reading unit 309 which is arranged at the upper side. Further, a flat bed portion 302 to be used when fixed-reading is performed on the original D is formed at the original base plate 301. A white reference plate 312 used as the white reference for the front face of the original D is arranged at the flat bed portion 302. Further, the optical reading unit 305 accommodates a lamp 307 and a CCD 306 and the optical reading unit 309 accommodates a lamp 311 and a CCD 310.
When double-side reading of the original D is performed at the abovementioned reading mechanism portion 300, the optical reading unit 305 is stopped at the position as illustrated in FIG. 7 after reading the white reference of the white reference plate 312 which is located at the right side in FIG. 7. Similarly, the optical reading unit 309 is to be on standby after reading the white reference of the platen 313 until the document D is fed.
When the originals D are continuously fed by the sheet feeding roller 303 toward the optical reading units 305, 309, reflecting light of the light irradiated from the lamp 307 is read by the CCD 306 in the optical reading unit 305 as one side. Further, reflecting light of the light irradiated from the lamp 311 is read by the CCD 310 in the optical reading unit 309 as the other side. In this manner, the reading of the upper and lower faces of the originals D is performed.
Next, the image signal processing circuit included in the image reading apparatus of the related art illustrated in FIG. 7 will be described with reference to FIG. 8. In FIG. 8, the image signal processing circuit to read the image of the front face is illustrated at the upper part and the image signal processing circuit to read the image of the back face is illustrated at the lower part. Since the image signal processing circuits for the front face and for the back face respectively have the similar configuration and processing nature, only the processing nature for the front face is described in the following and the description of the processing nature of the image signal processing circuit for the back face is not repeated.
The CCDs 306, 310 indicated in FIG. 7, amplifiers 321, 326, A/D conversion circuits 322, 327, white level correction circuits 323, 329 and memories 325, 330 are illustrated in FIG. 8. Here, for example, white level data for the white reference of the white reference plate 312 which is firstly read by the CCD 306 is stored at the white level correction circuit 323. Then, the outputted video signal after reading the original by the CCD 306 is amplified by the amplifier 321 and inputted to the white level correction circuit 323 and the A/D conversion circuit 322. In the white level correction circuit 323, a ground color level of the inputted video signal is detected, tracking and correcting of the white level are performed by means of the white level data stored at the white level correction circuit 323, and then, the white level is outputted to the A/D conversion circuit 322. The A/D conversion circuit 322, taking the white level received from the white level correction circuit 323 as a saturated value of darkness, converts the video signal of an analog signal received from the CCD 306 via the amplifier 321 into image data of a digital signal of corresponding darkness level. As described above, in the reading mechanism portion 300 in the image reading apparatus of the related art, the front and back faces of originals have been read and stored at the memories 325, 330 while performing the abovementioned conversion process (i.e., the correction process).
Among the apparatuses capable of reading both faces of originals as described above, there has been a configuration having two reading units. A first reading unit is movably mounted on a carrier which runs along an original base plate and a second reference color face for a second reading unit is arranged at the carrier. Then, the positional relation between a fixed first reference color face for the first reading unit and a reading window of the second reading unit is arranged to be symmetric with the positional relation between the second reference color face and a reading window of the first reading unit. Thus, in the double-side reading apparatus having the configuration that two reading units are opposed while sandwiching an original path, it is described that the configuration in which setting of the reference color level for the first and second reading units can be performed with simple operation is provided (see Japanese Utility Model Application Laid-open No. 05-74072).
Further, among the apparatuses capable of reading both faces of originals as described above, there has been a configuration that a white reference sheet is arranged at an original feeding path at a side opposed to a reading position of automatic feed reading of an optical reading unit. In the description of Japanese Patent Application Laid-open No. 05-83480, time required for reading a white reference in the automatic feed reading is shortened by constituting the optical reading unit to read the abovementioned white reference sheet right before an original is fed to the reading position in the automatic feed reading.
Furthermore, among the apparatuses capable of reading both faces of originals as described above, there has been a configuration that a second optical unit to read an original back face with a line image sensor is arranged at a vicinity of a reading position of an optical unit to read an original front face. In this apparatus, a first original base plate unit is constituted with a first white sheet and two clear plates nipping the first white sheet at a part thereof and a second original base plate unit is constituted with a second white sheet and two clear plates nipping the second white sheet at a part thereof. Then, the original base plate unit is to be compact by adopting a configuration that the first white sheet is opposed to the clear part of the second original base plate unit and the second white sheet is opposed to the clear part of the first original base plate unit (see Japanese Patent Application Laid-open No. 07-193680).
As described above, in the image reading apparatuses capable of reading both faces of originals such as the double-side image scanner of the related art, the white level has been generated by the optical reading units for front face reading and back face reading based on each white reference plate such as the white reference sheet and the white sheet.
However, there have been problems (1), (2) with the apparatuses to generate the white level as described above.
(1) In each of the optical reading units respectively for front face reading and back face reading, darkness correction for an intermediate color is not performed even in the case that the white level is appropriately adjusted. Therefore, there has been difference (i.e., deviation) of the reading darkness between the front face and the back face of a double-side original. In particular, for color reading, there is difference of reflecting ratio among color elements (R, G, B) even in the case that the white level is adjusted against the white reference plate.
Accordingly, there has been color difference of the read images. Consequently, there has been required to take costly measures such as performing white level setting based on the reflecting ratio measured for respective color elements (R, G, B) against a white level member of each part.
(2) The tone characteristics of the optical reading units for front face reading and back face reading are respectively varied due to variation (in accordance with durability) of light spectral characteristics of the lamp for mainly illuminating originals. Therefore, deviation of reading darkness between the front face and the back face of a double-side original has been occurred with time even in the case that the tone correction including intermediate colors of the optical reading units for front face reading and back face reading has been performed at factory shipment.
In the related art, there has been proposed a reading tone correction method to perform level-adjustment so that reading values of the front and back faces are matched by preparing a white reference sheet for testing having constant reflecting ratio at the front and back faces and by performing reading of the front and back faces (see Japanese Patent Application Laid-open No. 04-371072). With this proposal, although the problem (1) may be solved, the deviation of reading darkness between the front and back faces occurring as the problem (2) may not be solved. Further, increase in cost and expense in time for managing have been caused in view of preparing a special sheet for adjustment.